This invention relates to an improved correction lens adapted to form three colour phosphor elements of a fluorescent screen of a colour picture tube.
To form such phosphor elements, it is usual to use an exposure device generally termed a "lighthouse", consisting essentially of a light source, a member for supporting the face plate of a colour picture tube having a fluorescent screen and including a colour selection electrode, and a correction lens interposed between the light source and the face plate for causing the path of the exposure light emanated from the light source to approximate the electron beam in the colour picture tube during normal operation thereof. The electron beam lines during operation of the picture tube is greatly influenced by external magnetic fields including the terrestrial magnetism so that in order to have the path of the exposure light closely approximated the electron beam the optical surface of the correction lens should be constructed to have an extremely sophisticated configuration. This is caused by the fact that the aberration between the phosphor element and the electron beam spot varies greatly, that is the direction and magnitude of the aberration vary at random. For this reason, in order to correct such an aberration that vary from point to point it has been the practice to design the correction lens to divide it into numerous small lens segments and to establish a desired light path for each segment. In one design, the effective surface was devided in the form of a grid, whereas, according to another design the effective area was divided in the radial direction as disclosed in U.S. Pat. No. 3,279,340. In each design the design of the light path was made independently for each divided lens segment. Although such measures have succeeded to some extent to mate the path of the exposure light and that of the electron beam, design and manufacture of such correction lenses require a long duration and a large expense. Moreover, the correction lenses manufactured by these methods introduced serious problems. More particularly, as described hereinabove, since the design for each lens segment must be made independently it is inevitable that adjacent lens segment adjoin with a step shaped border lines therebetween. In other words, the number of step shaped border lines increases when the number of the divided lens segment is increased for the purpose of attaining more approximated path of the exposure light to the electron beam. Since each of surfaces surrounded with the step shaped border line shows different optical property for the projected light from that of the effective surface of the correction lens, uniform exposure of the fluorescent screen is impossible, thus forming a shadow. In this manner, a new problem of eliminating the shadow has arisen. The size of the phosphor element on the fluorescent screen is related directly to the energy of the exposure light and the size of the phosphor element is small at the shadow. Unless this problem is solved, it is impossible to uniformly form the phosphor elements. For the purpose of eliminating the shadow a number of proposals have been made including a method of vibrating the correction lens in a plurality of directions, a method of more finely dividing the elemental areas, a method of utilizing a dummy lattice (U.S. patent application Ser. No. 231,313, now U.S. Pat. No. 3,788,200, British Pat. No. 10114/72 and German Pat. No. P 22 10 725.1), a method of using a filter (U.S. patent application Ser. No. 231,219, now U.S. Pat. No. 3,809,558 British Pat. No. 10116/72, German Pat. No. P 22 10 724.0, and French Pat. No. 72 07519) and a method of utilizing a plurality of correction lenses, whose step shaped border lines are coated with an opaque substance, for performing a plurality of light exposure (U.S. Pat. No. 3,279,340). However, any one of these prior solutions added large cost to the sophisticated design and manufacturing of the correction lens described above. For example, the method of vibrating the correction lens requires to add means for vibrating the correction lens and means for controlling the vibration which of course increases the cost of installation and manufacturing. The other methods have similar disadvantages.